1. Field of the Invention
The invention relates to aircraft gas turbine engines with counter-rotatable low pressure turbines and, particularly, for such engines having geared together counter-rotatable low pressure turbines.
2. Description of Related Art
Conventional gas turbine engines of the turbofan type generally include a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some fan jet engines have been designed with counter-rotatable turbines that power counter-rotatable fans and boosters or low pressure compressors. U.S. Pat. Nos. 4,790,133, 4,860,537, 5,307,622 and 6,732,502 disclose counter-rotatable low pressure turbines (LPT) that power counter-rotatable fans and booster or low pressure compressors.
Counter-rotatable fans are noisy and to alleviate the noise problem and retain inherent efficiency of the counter-rotatable low pressure turbines, various designs have been developed with a single stage fan using gearboxes between the counter-rotatable low pressure turbines to drive the single stage fan. One such engine, described in U.S. Pat. No. 4,969,325, discloses counter-rotatable low pressure turbines having concentric shafts running forward to the fan and geared together by a gearbox in a fan section of the engine forward of the high pressure compressor of the core engine. The difficulty in design and construction and costs associated with two counter-rotatable low pressure concentric shafts running forward to the fan and geared together by a gearbox in a fan section of the engine forward of the high pressure compressor of the core engine is very well appreciated by those skilled in the art.
Another engine, described in U.S. Pat. No. 7,451,592, discloses single stage counter-rotatable low pressure turbines geared together by a gearbox in a rear section of the engine aft of the core engine and the aft of the single stage counter-rotatable low pressure turbines. A single shaft runs forward to power the fan. However, among the drawbacks of this design are the large exit areas of the single stage counter-rotatable low pressure turbines. The large exit areas of the single stage counter-rotatable low pressure turbines also requires large radius low pressure turbine sections which are also highly undesirable.